1. Field of the Invention
This invention relates generally to cryogenic storage systems and more particularly to a cryogenic fluid management system for zero gravity applications.
2. Description of the Related Art
Long term storage of sub-critical cryogens in space must address the problem of thermal stratification in storage tanks, and associated feed systems. Due to the absence of gravity induced body forces, thermal stratification in zero gravity can be much more severe than commonly experienced in a one G environment. If left uncontrolled, the thermal gradients in zero gravity will result in excessive tank pressure rise and formation of undesirable liquid and vapor mixtures within the liquid bulk, liquid acquisition system, and liquid transfer lines.
Under zero gravity conditions, gases in any storage tank for liquids may occur in any physical location within the interior of the tank, for example, either at the top of the tank, the bottom of the tank, or any intermediate section thereof.
A vehicle adapted to be subjected to the zero gravity operating environment of outer space may include a storage tank for liquids wherein solar heat input will increase the pressure of the tank and gases must be vented from the tank to prevent overpressurization and rupture. A special problem is thus presented in that it is necessary to vent only the gas from the system without loss of all liquid stored in the tank.
State of the art thermodynamic venting system concepts have only addressed issues associated with the tank. Typically, only the liquid bulk has been considered. Ullage, feedline, or zero gravity liquid acquisition devices (screens, capillary channels), have not been included in the total thermal destratification system design. Also, the performance of previous thermodynamic venting concepts has relied on complex analyses to characterize the zero gravity fluid mixing and heat transfer phenomenon.
U.S. Pat. No. 3,105,361, entitled "Zero Gravity Vent System", issued to L. R. Bell, discloses the utilization of a centrifuge to separate the liquid and vapor phases of the fluid withdrawn from a cryogenic storage tank which is subsequently used to generate power to compress the withdrawn two-phase fluid and also to chill the liquid fluid phase across a heat exchanger prior to returning it to the cryogenic storage tank. Pressure reduction in the cryogenic liquid tank in this concept always results in overboard fluid venting losses. The venting loss increases if the fluid straight from cryogenic liquid tank is primarily of liquid phase with the presence of liquid droplets in the overboard discharge flow.
U.S. Pat. No. 4,412,851, entitled "Cryogenic Apparatus Suitable for Operations in Zero Gravity" issued to R. Laine, discloses the utilization of a phase separator inside a storage tank to absorb heat from the bulk fluid through fluid evaporation heat transfer and subsequent fluid venting. The heat transfer rate between the phase separator and the liquid bulk is not enhanced above the zero gravity natural convection heat transfer mode. Consequently venting losses are high due to an inefficient internal tank heat transfer rate.
U.S. Pat. No. 3,693,367, entitled "Thermodynamic Control Device" issued to L. J. DiPeri, discloses the utilization of a surface tension device to establish a gaseous barrier between liquid inside a container and a tank wall. Complete structural isolation of liquid from a containment wall is maintained, thereby minimizing heat transfer. The DiPeri device is designed only to minimize the heat transfer to the liquid and does not address the issues associated with zero gravity venting or fluid management.
U.S. Pat. No. 3,304,729, entitled "Cryogenic Storage System" issued to W. A. Chandler et al, U.S. Pat. No. 3,699,696, entitled "Cryogenic Storage and Expulsion Means", issued to R. L. Rhoton and U.S. Pat. No. 4,821,907, entitled "Surface Tension Confined Liquid Cryogen Cooler", issued to S. H. Castles et al., also address the issues associated with minimizing heat leakage to cryogenic liquid storage systems.